Wavelength and temperature dependence of continuous-wave laser absorptance in Kapton thin films

Transcription

1 Wavelength and temperature dependence of continuous-wave laser absorptance in Kapton thin films William J. Palm Michael A. Marciniak Glen P. Perram Kevin C. Gross William F. Bailey Craig T. Walters

2 Optical Engineering 51(12), (December 2012) Wavelength and temperature dependence of continuous-wave laser absorptance in Kapton thin films William J. Palm Air Force Research Laboratory Kirtland AFB, New Mexico Michael A. Marciniak Glen P. Perram Kevin C. Gross William F. Bailey Air Force Institute of Technology Department of Engineering Physics Wright-Patterson AFB, Ohio Craig T. Walters Craig Walters Associates Powell, Ohio Abstract. Optical properties and laser damage characteristics of thin-film aluminized Kapton were investigated. Spectral absorptance of virgin and irradiated samples was measured from the Kapton side of multilayered insulation over 0.2 to 15 μm wavelengths at both room temperature and 150 C. The laser-damage parameters of penetration time and maximum temperature were then measured in a vacuum environment at laser wavelengths of 1.07 and 10.6 μm. Differences in damage behavior at these two wavelengths were observed due to differences in starting absorption properties at these wavelengths. During laser irradiation, the Kapton thin film was observed with a calibrated FLIR thermal imager in the 8 to 9.2 μm band to determine its temperature evolution. Spectral radiance throughout the mid- and long-wave infrared was also observed with a Fourier transform spectrometer, allowing temperature-dependent spectral emittance to be determined. Kapton emittance increased after the material heated past approximately 500 C, and continued to increase as it cooled posttest. This evolving temperature-dependent spectral emittance successfully predicts the increasing absorptance that led to shortened penetration times and increased heating rates for the 1.07 μm laser. For tests with constant absorptance and no material breakdown, a simplified one-dimensional thermal conduction and radiation model successfully predicts the temporally evolving temperature Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: /1.OE ] Subject terms: laser heating; Kapton ; polyimide; spectral emittance. Paper SSP received Mar. 15, 2012; revised manuscript received Apr. 25, 2012; accepted for publication Apr. 25, 2012; published online Jul. 10, Introduction Prediction of laser-target thermal interaction is complicated by the transient nature of the target material s properties as it undergoes laser irradiation. This research characterized the wavelength-dependent correlation between laser damage and initial laser absorptance by considering the primary mechanisms for damage. Remote-sensing equipment was used before, during, and after laser irradiance testing to analyze the transient temperature and absorption properties of thin-film aluminized Kapton in a vacuum environment. In this fashion, spectral data could be used to confirm evolving temperature results from calibrated FLIR thermal images and compare them to evolving absorption trends during material decomposition. As expected, laser absorptance was determined to be the greatest contributing factor to material heating, but it was also observed to vary greatly with laser wavelength and irradiance and depend on the extent of Kapton decomposition. 1 Absorptance measurements were made for the aluminized Kapton film before, during, and after irradiation at several laser irradiance levels. This work reports an independent spectral characteristic for each phase in the life of the irradiated material, yielding temperature- and wavelength-dependent absorptance. Along with a simple thermal model, this data may be useful for scaling bulk material thermal interactions between various laser /2012/$ SPIE wavelengths in other materials if the transient changes in absorptance in those materials are properly characterized. Remote-sensing instruments like the Fourier-transform spectrometer (FTS) serve as valuable tools in determining how laser coupling evolves and affects the prime damage mechanisms. 2 Experiment Thin-film samples of aluminized Kapton measuring 7.62 cm 2 were irradiated at wavelengths in the near infrared (NIR, 1.07 μm) (see Fig. 1) and long-wave IR (LWIR, 10.6 μm) (see Fig. 2). The film used for this experiment was constructed of a 50 μm-thick Kapton polyimide with a thin aluminized back surface less than or equal to 300 Å. The tests were fully diagnosed with a full listing of relevant diagnostic equipment. 2 A small fraction of the main beam was split off to form a diagnostic beam train for each laser. This diagnostic beam train is a known constant fraction of the beam power and is collected in an integrating sphere with a thermopile detector that is linear over six orders of magnitude with a 10-ms response time (shown in Figs. 1 and 2). The thermopile detector was calibrated daily to the power at the target plane using a Molectron power meter. This technique ensured that the effects of the optics and beam dumps were factored over a range of powers. Irradiance was reported during the dwell time on each sample using a constant area of cm 2 and Optical Engineering December 2012/Vol. 51(12)

3 Fig. 1 Fiber laser beam train with beam splitters at the beginning and end of the beam train to measure power and spatial beam profile. the calibrated power. The uncertainty is presented as the standard deviation of power measurements collected at 100 Hz during the dwell time. Spatial beam profile for the LWIR beam was determined using polymethylmethacrylate, a typical profiling material that linearly ablates. The NIR laser followed the same setup, with an additional beam splitter placed at the final optic to minimize errors and provide a true replica of the beam profile at the target plane. The replica beam was imaged with a back-illuminated CMOS CCD video camera (shown in Fig. 2) and recorded to ensure profile repeatability. 3 Figure 3 depicts instruments involved in analyzing the material interaction with reference to the target plane. Strict control was used to calibrate the spectral effects of the vacuum chamber windows for the exterior instruments by performing a full radiometric analysis of the path from the sample to the detector. 4 3 Results Before irradiance testing, spectral reflectance for virgin aluminized Kapton samples was performed from the Kapton side using a Varian Cary 5000 spectraphotometer and a Bomem MB157S FTS. The spectrum of the polyimide film at room temperature ( 25 C) is presented as spectral absorptance in Fig. 4 and is identical to that measured at 150 C. These results are similar to previously reported Kapton spectra. 5,6 The laser irradiance test data from the FLIR thermal imager were reduced using a MATLAB curve fit developed from calibration elements produced within the FLIR ThermaCAM RCal program in the 8 to 9.2 μm region, a relatively flat, high-absorptance/emittance region as shown in Fig. 4. The normalized interferograms from the FTS were imported into a MATLAB code that converted them into Optical Engineering December 2012/Vol. 51(12)

4 Fig. 2 CO 2 laser beam train with two beam dumps and diagnostic beam splitting. uncalibrated spectra. A linear relationship to known blackbody calibration curves was then developed to yield calibrated apparent spectra with real units of radiance. After taking into account the vacuum chamber window attenuation, true source radiance for both the spectral measurement and the thermal image was achieved. Figure 5 shows example spectra of aluminized Kapton irradiated at W cm 2 in the NIR. The midwave infrared (MWIR) radiance, collected by the FTS s InSb detector, is shown as a dashed curve, and the LWIR radiance, collected by the FTS s HgCdTe (MCT) detector, as a dotted curve. The Planckian envelope was generated using the FLIR-imager-inferred temperature ( 500 C here). This demonstrates the continuity achieved between the FTS and FLIR thermal imager when viewing the same sample spot. Note that at 500 C, the spectral features observed at room temperature and the relatively flat, high-emittance region of the LWIR remain. 4 Discussion The Kapton film responded as anticipated based on the pretest absorptance measurements at the two laser wavelengths. At 10.6 μm, Kapton is highly absorptive, as are most organic materials, and the heating rate was high. At 1.07 μm, Kapton is fairly transparent, and heating rates were much lower for the same irradiances. Temperature was recorded as a function of time for each sample tested, and before major damage or decomposition, showed good correlation to a simple onedimensional (1-D) conduction and radiation model. This model involved a balance between estimated absorption using derived values, 1-D conduction into the material, and radiation from the Kapton side only. Figure 6 presents the calibrated FLIR data for a 10.6 μm laser irradiance of 2.98 W cm 2 on the Kapton side of the aluminized Kapton film. Two curves correspond to two frame integration times that are acquired alternately during FLIR measurements. FLIR Data 0 presents the FLIR data Optical Engineering December 2012/Vol. 51(12)

5 Fig. 3 Plan view sketch inside the vacuum chamber showing beam entrance and various diagnostic equipment locations. Fig. 4 Spectral absorbance for 2-mil aluminized Kapton at room temperature measured from Kapton side. for a 3-ms integration time, which is valid for low temperatures before the detector saturates; the validity range for this curve is 40 C to 250 C. FLIR Data 1 presents the FLIR data for a 1-ms integration time and is valid for temperatures of 350 C to 700 C. To get a continuous curve for temperature as a function of time, it was assumed that the transition between 250 C and 350 C was smooth and continuous and could be represented by a parabolic splice, T ¼ t 2 þ t 4.044; (1) with T in C and t in seconds. The resulting continuous curve for temperature was then compared to a simple laser heating model. The irradiance tests were conducted in a vacuum test environment which negated convective heat transfer. In this environment, the dominant heat transfer mechanism is radiation, with very little conduction into the sample stand due to the material s low thermal diffusivity. Whereas the test environment complicated the experimental setup, the theory was greatly simplified by removing convection and only treating 1-D conduction within the material and radiation from it. Based on the thermal diffusivity of Kapton, the characteristic distance for heat diffusion during a 60-s run is about 2 mm. This and the appearance of the samples after testing confirmed that there was no significant radial heat conduction. However, thermal radiation from the front surface played a dominant role in the temperature history of the Kapton film at long run times with little Kapton decomposition. This is illustrated by the FLIR thermal imager data compared to the 1-D thermal model, with radiation loss shown in Fig. 7(a) for 1.07 μm laser irradiance of 7.31 W cm 2.A Kapton thermal emittance of 0.41 in the 1-D thermal model provides an excellent fit to the FLIR test data. The thermal conduction model is a simple 1-D explicit finite difference numerical solver with appropriate boundary Optical Engineering December 2012/Vol. 51(12)

6 Fig. 5 Calibrated source spectra of aluminized Kapton undergoing 1.07-μm laser irradiation at W cm 2. Blackbody envelopes of each detector created for comparison using a Planckian curve fit at a FLIR -inferred temperature of approximately 500 C. conditions. The front surface (Kapton side) was assumed to absorb heat equal to the irradiance multiplied by the absorptance that was determined from the room-temperature measurements. Thermal radiation at the front surface was simply assumed to follow the Stefan Boltzmann law with a constant emittance. This emittance was adjusted to achieve a reasonably good agreement with the data at long exposure times (near radiative equilibrium). No heat loss was assumed to occur from the aluminized back surface, which has low emittance. As can be seen by the straight dashed line in Fig. 7(a), the early heating rate was empirically determined to be 72 C s. This fit works well when there is no damage to the material. The absorptance value of used was derived from a fit to the early heating rate, where radiation was not significant. This derived absorptance agrees to within 16% of the room-temperature steady-state value for 1.07 μm found pretest (0.09, Fig. 4). The emittance fit value of 0.41 was lower than the manufacturer s specification for Kapton (0.71), 7 which is probably valid for room temperature or colder. The initial material properties for Kapton used in this analysis are presented in Table 1. A similar comparison is made in Fig. 7(b) for 10.6 μm laser irradiance of 2.98 W cm 2. Here, the agreement between the FLIR data and the 1-D model is not quite as good. Although irradiance is lower than in Fig. 7(a), higher absorptance at 10.6 μm drives the temperature higher. The early heating rate was empirically determined to be approximately 257 C s [straight dashed line in Fig. 7(b)], and the absorptance derived from the early heating rate of 0.67 agrees to within 27% of the low-temperature steady-state value at 10.6 μm (0.916, Fig. 4). The difference between the FLIR data and the 1-D model seen between 2 and 6 s may be an indication of an endothermic chemical reaction within the material that slows the heating. Kapton has a reported decomposition temperature of 525 C, a feature that was observed in several different test cases. 5 The permanent discoloration of Kapton may begin slightly below this temperature after prolonged exposure above around 400 C. (Note that this is not pyrolysis, or char formation which occurs above 650 C). The effective thermal emittance increased to 0.48 in this example, a faster heating scenario, based on the radiation-balance, conduction, and absorption model. This modeled emittance value for the Kapton layer at 650 C is not significantly different than that at 430 C [0.41, Fig. 7(a)] and will mostly depend on the level of discoloration in the material. The nature of the discoloration is not addressed here. The increased effective thermal emittance at higher temperatures used in the heating model is consistent with an increase in spectral emittance at temperatures above room temperature measured by the FTS. FTS measurements during laser irradiation captured the change in spectral emittance with temperature and extent of surface discoloration. Throughout a test, nearly constant spectral absorptance was observed (as in Fig. 4) until the point where the Kapton surface starts to change color and decompose. Then, the FTS-measured emittance increased in the MWIR and remained nearly constant in the LWIR. Owing to equipment Fig. 6 Typical calibrated FLIR data for two integration times. Curve FLIR Data 1 represents the 1-ms integration, whereas FLIR Data 0 the 3-ms integration. The splice function between the two curves is represented with the data fit equation. Optical Engineering December 2012/Vol. 51(12)

7 Fig. 7 Comparison of FLIR temperature data in a 1-D thermal model with front surface radiation loss for 1.07-μm (a) and 10.6-μm (b) laser tests. Kapton absorptance derived at 1.07 μm was and that at 10.6 μm was Kapton thermal emittance required for the fit was calculated to be 0.41 at 1.07 μm and 0.48 at 10.6 μm. Table 1 Initial properties of Kapton film. 5,8 Material property Value for Kapton Mass density (ρ) 1.42 g cm 3 Heat capacity (C p ) 1.09 J gc Thermal conductivity (k) W cm C Thermal diffusivity (κ) cm 2 s Absorptance at 10.6 μm from reflectance Absorptance at 1.07 μm from reflectance 0.09 Absorptance at 10.6 μm from initial heating data 0.67 Absorptance at 1.07 μm from initial heating data limitations, spectra in the NIR were not captured, but a significant rise in emittance can be discerned by comparing the material/laser interaction in terms of increasing heating rates. Figure 8 shows pretest, real-time, and posttest spectral emittance in the MWIR. The real-time (FTS InSb-detector based) emittance flattened out in the MWIR region and then greatly increased after the sample was allowed to cool. Figure 9 shows the pre- and posttest room-temperature Kapton spectral emittances from the NIR through the LWIR. (Real-time data over this broader wavelength range was not available.) Although the data in Fig. 9 do not include the remote-sensing emittance data, an increase in MWIR absorptance was observed before cooling in the cases where material discoloration or decomposition began. By extrapolating heating rate data between the two laser wavelengths tested and normalizing the effects of absorption in the regime before radiation becomes significant, we can infer a rise in emittance for the 1.07 μm laser wavelength region of about 400%; that is, a starting material absorptance of 0.09 would Optical Engineering December 2012/Vol. 51(12)

8 Fig. 8 Comparison of pretest (room-temperature virgin) spectral emittance, spectral emittance measured during experimental decomposition (FTS InSb detector), and posttest (room-temperature decomposed) spectral emittance for a decomposed Kapton sample. Fig. 9 Comparison of aluminized Kapton spectral absorptance before and after laser irradiation to the point of decomposition. increase to 0.4 after the material reached 500 C and decomposed. After the Kapton is allowed to cool, the decomposed absorptance at 1.07 μm has increased by roughly 10 times its virgin value. An empirical trend for aluminized Kapton absorptance at increased temperatures due to laser irradiation can be established using the real-time FTS data in the range of 3 to 4 μm, where instrument gain is high and significant emittance changes were observed. Figure 10 plots 3.8 μm absorptance as a function of temperature and shows a slightly increasing absorptance during testing, with a marked increase as the decomposed material cools back to room temperature afterward. The absorptance variability during testing stems from the uncertainties in calibration and temperature extraction. Although decomposition did occur near 500 C, the material experienced a significant absorptance rise at 3.8 μm after cooling. A fully decomposed (blackened) sample at room temperature had an absorptance of 0.63 at 3.8 μm. Fig. 10 Empirical 3.8-μm absorptance of aluminized Kapton derived from FTS spectra captured during laser irradiation. Absorptance increases during the heating process between approximately 0.2 and 0.4, and then increases to 0.63 in the decomposed (blackened) sample after it is cooled back to room temperature. 5 Conclusion These results demonstrate the unique wavelength- and temperature-dependent material response that can be captured with remote sensing techniques such as those used here. This more-accurate characterization of the sample s spectral absorptance response then allows the scaling of laser test results between laser wavelengths by using simple modified heating models that better capture the true laser damage. By using only initial heating rates from a calibrated FLIR thermal imager and a simple 1-D thermal conduction model, a reasonable measure of material absorptance for aluminized Kapton was achieved at two laser wavelengths, 1.07 and 10.6 μm. FTS measurements demonstrated that trends in a material s spectral emittance may be captured for nondestructive laser/material interactions using remote sensing techniques. Here, emittance was most accurately captured in the MWIR, so absorptance trends for aluminized Kapton were charted at 3.8 μm. These results were consistent during nondestructive testing and could be duplicated at any wavelength in the MWIR. After a laser test, when the samples were allowed to cool back to room temperature, aluminized Kapton absorptance increased dramatically. The goal now is to use this varying-absorptance data along with the thermal model to produce more-accurate predictions of real-world transient conditions. Based on the data acquired here, it should be possible to estimate the thermal response of the Kapton to laser irradiation at wavelengths anywhere between 1 and 13 μm with greater accuracy than was possible prior to this investigation. Acknowledgments This work was supported by the High Energy Laser Joint Technology Office (HEL JTO) under grant AFOSR-BAA and was performed at Wright-Patterson AFB, OH. References 1. D. L. Decker, Temperature and wavelength dependence of the reflectance of multilayer dielectric mirrors for infrared laser applications, Laser Induced Damage in Optical Materials Proceedings Sponsored by the National Bureau of Standards, NBS-SP-435, (1975). 2. W. J. Palm et al., Laser induced damage of Kapton thin films demonstrating temperature and wavelength dependent absorptance: a case Optical Engineering December 2012/Vol. 51(12)

9 study in remote sensing material analysis, Proc. SPIE 8190, (2011). 3. C. T. Walters et al., High Energy Laser (HEL) Lethality Data Collection Standards, Directed Energy Professional Society, Albuquerque, NM (2007). 4. W. J. Palm, Multilayer insulation laser damage characterization for wavelength scaling, MS thesis, Air Force Institute of Technology (2011). 5. W. N. Pollard and B. Hannas, Non-contact temperature measurement of aluminized polymer for space applications, James Madison University, Infrared Development and Thermal Testing Lab, VA (2002). 6. M. J. Rowley, Fiber-optic infrared measurement system for thermal measurement of a Kapton HN sample, James Madison University, Infrared Development and Thermal Testing Lab, VA (2003). 7. Dupont Kapton, Polyimide Film General Specifications, www2.dupont.com/kapton/en_us/assets/downloads/pdf/gen_specs.pdf. 8. Product Data Specifications Sheet: Item No. MO09176, DE330, DE332, Dunmore Corporation (2007). William J. Palm currently leads research and development of high-brightness fiber-based laser systems and components for military applications on behalf of the Air Force Research Laboratory s Directed Energy directorate. He earned a BS in mechanical engineering from the University of Illinois and a MS in applied physics from the Air Force Institute of Technology. He has published scientific work related to both lasers and spacecraft testing through the Directed Energy Professional Society, SPIE, AIAA, and the Aerospace Corporation. Michael A. Marciniak received a BS degree in mathematics-physics from St. Joseph s College, IN, in 1981, a BSEE degree from the University of Missouri-Columbia in 1983, and an MSEE (electro-optics) and PhD (semiconductor physics) degrees from the Air Force Institute of Technology (AFIT) in 1987 and 1995, respectively. He is an associate professor in the Department of Engineering Physics at AFIT, with research interests in various aspects of light-matter interaction, including polarimetric scatterometry and thermal radiation of nanostructured materials, optical signatures, and high-energy-laser damage assessment. Kevin C. Gross graduated from the Air Force Institute of Technology (AFIT) with a PhD degree in physics in He joined the AFIT faculty in 2008 and is currently is an assistant professor. He runs the AFIT Remote Sensing Group and has been involved in the collection of high-speed radiometric, imagery, and spectroscopic measurement of battle space combustion signatures including high-explosive detonations, muzzle flashes, rocket engines, and jet engine exhaust plumes. William F. Bailey received a BS degree from the United States Military Academy in He received an MS degree in nuclear physics from the Ohio State University in 1966 and a PhD from the Air Force Institute of Technology (AFIT) in As a member of the AFIT faculty since 1978, his research interests include high-power lasers and microwave systems, plasma dynamics and diagnostics, and characterization of hypersonic aerodynamic flows. Craig T. Walters holds a PhD in physics from the Ohio State University and has performed extensive research in the area of laser effects on materials. He currently has his own consulting firm, Craig Walters Associates, a small business devoted to providing R&D services and consultation to industry and government in the areas of laser technology and electro-optics. He has performed contract research for other small businesses as well as multibillion-dollar corporations in laser application areas as diverse as laser cleaning and coating removal, laser beam diagnostics, optical system design, laser shock processing, laser-based inspection of adhesive bonds, laser-welding monitors, and high-power optical beam delivery systems. Prior to forming his own company, he had a distinguished thirty-year career at Battelle Columbus Laboratories, culminating in eight years of service as a Research Leader. Glen P. Perram received his BS degree from Cornell University in 1980 and his MS and PhD degrees from the Air Force Institute of Technology (AFIT) in 1981 and 1986, respectively. As professor of physics at AFIT, his research interests include high-power gas lasers, remote sensing, and laser-material interactions. Optical Engineering December 2012/Vol. 51(12)